This invention relates to new materials for photovoltaic devices and more specifically multiband semiconductors for high power conversion efficiency solar cells.
Various materials that are suitable for photovoltaic devices are known, such as tetrahedral amorphous semiconductors (e.g., amorphous silicon, amorphous silicon germanium and amorphous silicon carbide) as well as poly- and mono-crystalline semiconductors including group IV (Si), II-VI compound semiconductors, (e.g., CdTe), and III-V group compound semiconductors. (e.g., GaAs, GaInP, GaAlAs). A conventional solar utilizes the pn junction formed by ion implantation or thermal diffusion of impurities into a substrate of single crystal of silicon (Si) or gallium arsenide (GaAs), or by epitaxial growth of an impurity-doped layer on a substrate of such single crystal. However, such single junction solar cells have only limited efficiency because they are sensitive to a limited part of the total solar spectrum. The efficiency can be improved by using stacks of p/n junctions formed with semiconductors with different energy gaps that are sensitive to different parts of solar spectrum. This concept has been realized in multijunction or tandem solar cells (J. M. Olson, T. A. Gessert, and M. M. Al-Jasim, Proc. 18th IEEE Photovoltaic Specialists Conference, 552, Las Vegas, Oct. 21-25, 1985, the contents of which are incorporated by reference in its entirety) such as GaAs/GInP double junction or Ge/GaAs/GaInP triple junction cells. Power conversion efficiencies of 37% have been achieved in the most advanced versions of such cells. The complexity of the design and high fabrication costs limit the use of such cells to space applications (M. Yamaguchi, Solar Energy Mat. & Solar Cells, 75, 261 (2003).).
Another approach to improve the efficiency of solar cells has been based on the concept of multiband semiconductors (M. Wolf, Proc. IRE, 48, 1246 (1960) and A. Luque and A. Marti., Phys. Rev. Lett., 78, 5014 (1997). It has been postulated that instead of using several semiconductors with different band gaps one could use a single semiconductor with several absorption edges that absorb photons from different parts of the solar spectrum. The most important advantage of this design of high efficiency solar cells is that they require only a single p/n junction considerably simplifying the cell design and lowering the production costs. It has been theoretically predicted that ideal power conversion efficiencies up to 63% and 72% could be achieved in solar cells fabricated using materials with optimized three and four energy bands, respectively.
Practical realization of a multiband semiconductor that could be used for solar cells has turned out to be extremely difficult. There were several attempts to intentionally introduce large concentrations of impurities or defects that would form an additional narrow band in the band gap of a standard semiconductor such as Si or GaAs. These attempts were not successful as the impurities and defects changed the key electrical properties of the materials making preparation of properly operating solar cells impossible. To date there has been no confirmed demonstration of an operational solar cell based on the concept of multiband semiconductors.
Recently a new class of semiconductors has emerged, whose fundamental properties are dramatically modified through the substitution of a relatively small fraction of host atoms with an element of very different electronegativity, the so called highly mismatched alloys (HMAs). III-V alloys in which group V anions are partially replaced with the isovalent N [Semiconductor Science and Technology 17, 2002, Special Issue: III-N-V Semiconductor Alloys, the contents of which are hereby incorporated by reference in its entirety] or II-VI alloys in which group VI anions are partially replaced with O [K. M. Yu, W. Walukiewicz, J. Wu, J. W. Beeman, J. W. Ager, E. E. Haller, I. Miotkowski, A. K. Ramdas, and P. Becla, Appl. Phys. Lett. 80, 1571 (2002), the contents of which are hereby incorporated by reference in its entirety,] are the well known examples of the HMAs. For example, GaNxAs1-x exhibits a strong reduction of the band gap by 180 meV when only 1% of the As atoms is replaced by N. It has been predicted and experimentally demonstrated that the electronic band structure of such HMAs is determined by the anticrossing interaction between localized O or N states and the extended states of the semiconductor matrix [W. Walukiewicz, W. Shan, K. M. Yu, J. W. Ager III, E. E. Haller, I. Miotlowski, M. J. Seong, H. Alawadhi, and A. K. Ramdas, Phys. Rev. Lett. 85, 1552 (2000), the contents of which are hereby incorporated by reference in its entirety]. The interaction splits the conduction band into two nonparabolic subbands: E+ and E−.